In general, the present invention relates to a method of driving a liquid-crystal display device and the liquid-crystal display device driven by the method. More particularly, the present invention relates to an active-matrix liquid-crystal display device having a driving frequency decreased to lower its power consumption and a method of driving the liquid-crystal display device.
Japanese Patent Laid-open No. 2002-182619 discloses a method of driving an active-matrix type liquid-crystal display device in order to reduce its power consumption. The method disclosed in this reference has a scanning period and a break period longer than the scanning period. The scanning period is a period during which a screen is scanned. The break period is a period in which all gate lines are put in an off state. In addition, in accordance with this method, by setting the electric potential of data lines at a predetermined signal-line break level during the break period, the frame frequency can be reduced from the present frequency of 60 Hz to decrease the power consumption.
In general, if the frame frequency is reduced, extremely small screen changes, which cannot be sensed at a frequency of 60 Hz so far, becomes noticeable by the sense of sight as the so-called flickers. The extremely small screen changes are caused by changes in transmittance for transmission and transflective types and changes in reflectance for reflection and transflective types. There exist flickers generated by a variety of causes according to various kinds of driving. Flickers generated by leakage currents synchronously with the frame frequency exist in the so-called driving method using an active device. The leakage currents include a leakage current flowing in an off state of the active device and a leakage current of a liquid-crystal layer. The leakage current flowing in an off state of the active device is referred to hereafter as an off-state leakage current. The more the frame frequency is reduced, the more easily the flickers are sensed. In addition, flickers caused by a dc element also exist. When the electric potential of a data line changes, the electric potential of the pixel electrode of a pixel connected to the data line in an off state changes due to capacitive coupling between the data line and the pixel electrode of the pixel. In turn, the changes in electric potential appearing on the pixel electrode cause such flickers.
The conventional driving method does not adequately cope with flickers caused by a leakage current of a liquid-crystal layer and an off-state leakage current of an active device. Flickers caused by a leakage current of a liquid-crystal layer and an off-state leakage current of an active device cannot be made unnoticeable by the sense of sight of a human being by making the transmittances or the reflectances of adjacent pixels cancel each other. That is to say, it is impossible to make the transmittances or the reflectances of adjacent pixels cancel each other by carrying out operations such as column inversion driving, row inversion driving or dot inversion driving as is the case with flickers caused by a dc element. Thus, flickers caused by a leakage current of a liquid-crystal layer and an off-state leakage current of an active device are flickers existing for all driving methods independently of the types of the driving methods as flickers generated synchronously with frame periods.
For the reasons described above, reduction of screen changes caused by transmittance and reflectance changes due to a leakage current of a liquid-crystal layer and an off-state leakage current of an active device can be considered to be a requirement indispensable to driving at a low frequency while sustaining a high picture quality. It is to be noted that the scope of the present invention is not limited to a liquid-crystal display device using a liquid-crystal panel of the so-called vertical electric-field type wherein a pair of substrates is used. On one of the substrates, gate lines, data lines and active devices are created whereas, on the other substrate, opposite electrodes are created. An example of the vertical electric-field type is a TN mode. For example, the present invention can also be applied in the same way to a liquid-crystal display device using a liquid-crystal panel of the so-called horizontal electric-field type (or an IPS mode) also using a pair of substrates and a liquid-crystal display device of another known active-matrix type. In the liquid-crystal display device using a liquid-crystal panel of the so-called horizontal electric-field type, opposite electrodes are created also on one of the substrates, which is used for creating gate lines, data lines and active devices. Thus, the phrase stating: “holding a liquid-crystal layer in a state of being sandwiched by pixel electrodes and opposite electrodes” in this specification means that a liquid-crystal layer exists between pixel and opposite electrodes on one of a pair of substrates for the IPS mode.
In addition, the present invention can also be applied to a liquid-crystal display device using a transmission-type liquid-crystal panel in which an illumination light beam incoming from a source outside one of a pair of substrates is radiated out from the other substrate. Furthermore, the present invention can also be applied to a liquid-crystal display device using a reflection-type liquid-crystal panel in which an illumination light beam incoming from a source outside one of a pair of substrates is radiated out from the same substrate. Moreover, the present invention can also be applied to a liquid-crystal display device using a transflective-type liquid-crystal panel in which both a reflection display unit and a transmission display unit are employed.